Chemical attachment of the hydrophilic polymer poly(ethylene glycol)(PEG), which is also known as poly(ethylene oxide) (PEO), to molecules and surfaces is of great utility in biotechnology. In its most common form PEG is a linear polymer terminated at each end with a hydroxyl group:HO—CH2CH2O—(CH2CH2O)n—CH2CH2—OHThis polymer can be represented in brief form as “HO-PEG-OH” where it is understood that the -PEG- symbol represents the following structural unit:—CH2CH2O—(CH2CH2O)n—CH2CH2—,where n typically ranges from approximately 10 to approximately 2000.
PEG is commonly used as methoxy-PEG-OH, or “mPEG”, in which one terminus is the relatively inert methoxy group, while the other terminus is a hydroxyl group that is subject to ready chemical modification.CH3O—(CH2CH2O)n—CH2CH2—OH mPEG
PEG is also commonly used in branched forms that can be prepared by addition of ethylene oxide to various polyols, such as glycerol, pentaerythritol and sorbitol. For example, the four-arm, branched PEG prepared from pentaerythritol is shown below:C(CH2—OH)4+nC2H4O→C[CH2O—(CH2CH2O)n—CH2CH2—OH]4 
Such branched polyethylene glycols can be represented in general form as R(-PEG-OH)z in which R represents the central “core” molecule, such as glycerol or pentaerythritol, and z represents the number of arms extending therefrom.
PEG is a well-known polymer having the following properties: solubility in water and in many organic solvents, lack of toxicity, and lack of immunogenicity. One use of PEG is to covalently attach the polymer to insoluble molecules to make the resulting PEG-molecule conjugate soluble. For example, Greenwald, Pendri and Bolikal in J. Org. Chem., 60, 331-336 (1995), have shown that the water-insoluble drug, taxol, when coupled to PEG, becomes water-soluble.
In related work, Davis et al., in U.S. Pat. No. 4,179,337, have shown that proteins coupled to PEG have enhanced blood-circulation lifetimes due to reduced rate of kidney clearance and reduced immunogenicity. Hydrophobic proteins have been described that, upon coupling to PEG, have increased solubility in water. These and other uses for PEG are described in J. M. Harris, Ed., “Biomedical and Biotechnical Applications of Polyethylene Glycol Chemistry,” Plenum, New York, 1992).
To couple PEG to a molecule such as a protein or onto a surface, it is necessary to use an “activated derivative” of the PEG having a functional group at a terminus suitable for reacting with some group on the protein or on the surface (such as an amino group). Among the many useful activated derivatives of PEG is the succinimidyl “active ester” of carboxymethylated PEG as disclosed by K. Iwasaki and Y. Iwashita in U.S. Pat. No. 4,670,417. This chemistry can be illustrated with the active ester reacting with amino groups of a protein (the succinimidyl group is represented as “NHS” and the amino-containing protein is represented as PRO-NH2):PEG-O—CH2—CO2—NHS+PRO-NH2→PEG-O—CH2—CO2—NH-PRO
Certain problems have arisen in the art. Some of the functional groups that have been used to activate PEG can result in toxic or otherwise undesirable residues when used for in vivo drug delivery. Some of the linkages that have been devised to attach functional groups to PEG can result in an undesirable immune response. Additionally, certain functional groups do not have appropriate selectivity for reacting with particular groups on proteins and can deactivate the proteins when in conjugated form.
Attachment of a PEG derivative to a substance can have a somewhat unpredictable impact on the substance. Proteins, small drugs, and the like may have reduced biological activity when conjugated to a PEG derivative, although in some cases, activity may be increased.
Another example of a problem that has arisen in the art is exemplified by the succinimidyl succinate active ester, “mPEG-SS” (the succinimidyl group is represented as NHS):

The mPEG-SS active ester is useful for coupling because it reacts rapidly with amino groups on proteins and other molecules to form an amide linkage (—CO—NH—). A problem has been reported with the mPEG-SS active ester, as noted in U.S. Pat. No. 4,670,417. Since this compound possesses an ester linkage in the backbone that remains after coupling to an amine group, such as in a protein (represented as PRO-NH2):mPEG-SS+PRO-NH2→mPEG-OC(O)—CH2CH2—CONH-PRO,the remaining ester linkage is subject to rapid hydrolysis and detachment of PEG from the modified protein. Too rapid a hydrolysis rate can preclude use of a PEG derivative for many applications. Several approaches have been adopted to solve the problem of hydrolytic instability. For example, mPEG succinimidyl carbonate has been proposed, which contains only ether linkages in the polymer backbone and reacts with proteins to form a conjugate that is not subject to hydrolysis.
In view of the above, it would be desirable to provide alternative PEG derivatives that are suitable for drug delivery systems, including delivery of proteins, enzymes, and small molecules, or for other biotechnology uses. It would also be desirable to provide alternative PEG derivatives that could enhance drug delivery systems or biotechnology products.